1. Field of the Invention
This invention relates to a method of furnace operation and apparatus in which a gas seal is used, in particular where gas turbine exhaust (GTE) is employed as preheated combustion air for burning the fuel in the furnace burners.
The furnaces contemplated may be used for various hydrocarbon conversions such as the steam reforming of natural gas or light hydrocarbons by steam to hydrogen and carbon oxides, which is especially useful in ammonia plants; and in steam cracking to convert petroleum fractions such as ethane up to naphtha and gas oil to lighter products, especially C.sub.2 -C.sub.4 olefins and heavier. In steam cracking the feed is generally mixed with from about 20 to 92 mol % steam and heated to temperatures in the range of about 1200.degree. to 1800.degree. F.
The operation of such furnaces requires the expenditure of large amounts of energy and increasingly is becoming the object of study in order to reduce fuel usage.
One approach to this is to use preheated combustion air in burning the fuel. For example, the exhaust gas from gas turbines may be used. Turbines of this type find use in many plants for supplying power, for example in steam cracking plants for driving olefin gas compressors and/or refrigerant compressors. The gas turbine exhaust is generally available at temperatures in the range of about 400.degree. to about 1000.degree. F. and contains oxygen in the range of about 15 to about 20 weight %.
One problem that arises in reusing the air in the exhaust from a gas turbine--which is a by-product of its operation--is that the turbine is run in accordance with its own schedule, viz., its own duty, thus may be shut off at times with consequent interruption of the flow of GTE to furnaces utilizing it as combustion air. Conversely, before start-up or on shutting down a furnace or one or several of a multi-furnace plant, the flow of GTE from the turbine is not needed.
According to the present invention an apparatus and method wherein a gas seal is used, are provided in which the furnaces are uncoupled from the gas turbine in the sense that the furnaces may be operated independently of the turbine and the turbine may be operated independently of the furnaces. Additionally, any one of a group of furnaces may be operated independently of the others in the group.
In the event that a gas turbine trips, one concern is that the furnace may become starved for oxygen temporarily. The resulting build-up of a high concentration of hydrogen or hydrocarbons in the furnace atmosphere followed by a sudden surge of oxygen, has the potential for causing an explosion and fire. An advantage of the gas seal of the invention is its rapid response time. When a gas turbine trips, the seal operates rapidly to admit cold air from the atmosphere, which it has up to then blocked off, and allow it to flow to the furnace burners as combustion air. This is an excellent safety feature.
2. Description of the Prior Art
The use of GTE as preheated combustion air in steam reforming furnaces is known from the following publications
U.S. Pat. No. 3,424,695 issued Jan. 28, 1969 to von Wiesenthal.
U.S. Pat. No. 3,795,485 issued Mar. 5, 1974 to Bogart (Flour Corp.)
"Hydrocarbon Processing," Apr. 1978, p. 145-151, by Bogart.
U.K. Pat. No. 1,200,227 published on July 27, 1970, is also of some interest.
However, these workers have not addressed the problem of uncoupling the gas turbine from the furnace. Neither the gas seal of this invention nor any other means is shown for permitting them to be operated independently of each other.
In U.S. Pat. No. 4,085,708 issued on Apr. 25, 1978, to Ashdown (Foster Wheeler Energy Corp.) this problem is considered in connection with running a waste heat steam boiler but it may be noted that at all times a primary source of air supplied by a forced draft fan is used for at least some of the combustion air needs. As regards the secondary source of air, it is supplied either by gas turbine exhaust or by air supplied by a second forced draft fan, the switching being controlled by the opening or closing of mechanical obstructing means. See also U.S. Pat. Nos. 3,301,223 and 3,118,429.
U.S. Pat. No. 3,789,804 discloses a method of connecting the exhaust of a second gas turbine to the burners of a steam generator, if a first one should fail. It may be noted that two turbines have to be available and that a gas turbine has to be in operation at all times so that the system is not really uncoupled. Furthermore, the switchover is accomplished by means of valves.